Carbazole photoinitiators for the photopolymerization of unsaturated compounds

ABSTRACT

Photopolymerizable prepolymer compositions are provided comprising an unsaturated compound having at least one ethylenic double bond that is photopolymerizable to form a polymer, and a photoinitiator having one of the formulae: ##STR1##

Photochemical polymerization processes have attained substantialimportance in the art, especially in those cases where thin layers haveto be hardened in a short time, for example, in the hardening of varnishcoatings, or in the drying of printing inks.

Many compounds are known as photoinitiators for the photopolymerizationof unsaturated compounds. Aromatic ketones such as benzophenone are mostcommonly used, due to their excellent solubility to unsaturatedcompounds. Recently, α-substituted aromatic aliphatic ketones wereproposed as photoinitiators in U.S. Pat. No. 4,308,400, andsulfur-containing-α-amino-aromatic-aliphatic ketones were proposed asphotoinitiators in Japan Kokai No. 83-157805.

Many of these known photoinitiators have the shortcoming that when mixedwith an unsaturated compound they display an insufficient storage life.Furthermore, these photoinitiators tend to cause an undesirableyellowing of the polymerized composition, and they are insufficientlyreactive, as evidenced by a relatively long polymerization time. Someare rapidly inactivated by atmospheric oxygen. There is therefore a needin the art for photoinitiators which are readily soluble in thesubstrate, and which, having a good storage life in the dark, initiatethe photopolymerization more rapidly and give a higher polymer yield perunit of time, than the known photoinitiators.

In accordance with this invention, carbazole photoinitiators areprovided having the following formulae (I), (II) and (III) that possessthe required properties as photoinitiators. In particular, they effect arapid photopolymerization, and either do not display the shortcomingsreferred to, or display them to a much lesser degree, than the knownphotoinitiators. Furthermore, they are suitable for the photochemicalcrosslinking of polyolefins. ##STR2## wherein: R₁ and R₂ are each alkylhaving from one to about eighteen carbon atoms; or are taken together toform alkylene having from about two to about twelve carbon atoms;

R₃ is selected from the group consisting of hydrogen; alkyl having fromone to about eighteen carbon atoms; and acyl having from about two toabout twelve carbon atoms;

X₁ is --OR₅ or ##STR3## in which R₅ is selected from the groupconsisting of hydrogen; alkyl and alkenyl having from one to abouteighteen carbon atoms; and R₆ and R₇ are selected from the groupconsisting of alkyl; hydroxyalkyl having from one to about eighteencarbon atoms; and R₆ and R₇ taken together to form alkylene having fromabout two to about twelve carbon atoms; and oxadialkylene andiminodialkylene having from four to about twenty-four carbon atoms;

Y₁ is hydrogen or ##STR4## Y₂ is selected from the group consisting ofhydrogen, halogen, and nitro;

R₄ is a direct linkage (--) or alkylene having from about two to abouttwelve carbon atoms;

X₂ is --O--R₈ --O-- or ##STR5## in which: R₈ is alkylene having fromabout two to about twelve carbon atoms;

R₉ and R₁₀ are alkyl having from one to about eighteen carbon atoms; orare taken together to form alkylene having from about two to abouttwelve carbon atoms.

Exemplary R₁, R₂, R₃, R₅, R₆, R₇, R₉ and R₁₀ alkyl are methyl, ethyl,propyl, isopropyl, butyl, sec-butyl, t-butyl, isobutyl, amyl, sec-amyl,isoamyl, tert-amyl, hexyl, isohexyl, sec-hexyl, tert-hexyl, heptyl,isoheptyl, tert heptyl, sec-heptyl, octyl, isooctyl, 2-ethylhexyl,nonyl, isononyl, tert-nonyl, decyl, undecyl, dodecyl, tridecyl,tetradecyl, heptadecyl and octadecyl.

Exemplary R₁ and R₂ taken together as alkylene include ethylene,propylene, butylene, amylene, hexylene heptylene, octylene, nonylene,decylene and dodecylene.

Exemplary R₃ acyl include acetyl, propionyl, butyroyl, valeroyl,caproyl, 2-ethylhexanoyl, decanoyl and lauroyl; acryloyl, methacryloyl,vinylacetyl, benzoyl, toluoyl, t-butylbenzoyl and chlorobenzoyl.

Exemplary R₅ alkenyl include allyl and octadecenyl.

Exemplary R₆ and R₇ hydroxyalkyl include hydroxyethyl, 2-hydroxypropyland 2-hydroxybutyl, 2-hydroxyhexyl, 2-hydroxyoctyl, 2-hydroxynonyl, and2-hydroxydodecyl.

Exemplary R₆ and R₇ alkylene, oxadialkylene and iminodialkylene includeethylene, propylene, butylene, amylene, hexylene, heptylene, octylene,decylene and dodecylene, oxadiethylene and iminodiethylene.

Exemplary Y₂ halogen include chlorine, bromine, fluorine and iodine.

Exemplary R₄ alkylene include methylene, ethylene, trimethylene,tetramethylene, hexamethylene, octamethylene, decamethylene anddodecamethylene.

Exemplary R₈ alkylene include ethylene, propylene,2,2-dimethylpropylene, 1,4-butylene, hexamethylene and decamethylene.

Exemplary R₉ and R₁₀ alkylene include methylene, ethylene, propylene,butylene, amylene, hexylene, octylene, decylene and dodecylene.

The following are carbazole compounds of the formulae (I), (II) and(III) falling within the invention.

1. 3-(2-methyl-2-dimethylaminopropionyl) carbazole

2. 3-(2-methyl-2-morpholinopropionyl)-9-methylcarbazole

3. 3,6-bis(2-methyl-2-morpholinopropionyl)-9-methylcarbazole

4. 3-(2-methyl-2-morpholinopropionyl)-9-butylcarbazole

5. 3-(2-ethyl-2-piperidinopropionyl)-9-butylcarbazole

6. 3-(2-methyl-2-diethanolaminopropionyl)-9-methylcarbazole

7. 3-(2-methyl-2-dibutylaminopropionyl)-6-chlorocarbazole

8. 3-(2-methyl-2-piperidinopropionyl)-6-nitro-9-methylcarbazole

9. 3-(2-methyl-2-morpholinopropionyl)-9-acetylcarbazole

10. 3,6-bis(2-methyl-2-morpholinopropionyl)-9-benzoylcarbazole

11. 3-(2-methyl-2-hydroxypropionyl)-9-methylcarbazole

12. 3-(2-methyl-2-methoxypropionyl)-9-methylcarbazole

13. 3-(2-methyl-2-allyloxypropionyl)-9-methylcarbazole

14. 3-(1-hydroxycyclohexanoyl)-9-butylcarbazole

15. 1,4-bis(9-butyl-3-carbazolyl)-2,3-dihydroxybutane-1,4-dione

16. 1,6-bis(9-methyl-3-carbazolyl)-2,5-bis(diethylamino)hexane-1,6-dione

17. 1,4-bis(1-(9-methyl-3-carbazolyloyl) isopropyl) piperidine

18. N,N'-dimethyl-N,N'-bis(1-(9-methyl-3-carbazolyloyl) isopropyl)ethylenediamine

19. 2,7-bis(1-(9-butyl-3-carbazolyloyl)isopropyl)-2,7-dimethyl-3,6-dioxaoctanol

The carbazole compounds of formulae (I), (II) and (III) can be readilyprepared by known procedures, for example, by the method described inU.S. Pat. No. 4,308,400.

The compounds of formulae (I) and (III) can be prepared by brominatingthe compound shown below, reacting the resulting compound with H--X₁ orH--X₂ --H: ##STR6##

The compounds of the formula (II) can be prepared by brominating thecompound shown below, reacting the resulting compound with H--X₁.

The following example is illustrative:

EXAMPLE I Synthesis of3-(2-methyl-2-morpholinopropionyl)-9-methylcarbazole ##STR8##

3-(2-Methylpropionyl)-9-methylcarbazole 15 g was dissolved in 50 ml ofcarbon tetrachloride. A solution of 12 g of bromine in 30 ml of carbontetrachloride was added dropwise over one hour at room temperature, andthe reaction mixture then stirred for one additional hour at roomtemperature. Then nitrogen gas was introduced to remove the hydrogenbromide produced and excess bromine.

Morpholine 30 g was added to the reaction mixture, which was then heatedup to 120° C., while carbon tetrachloride was distilled off. Thesolution was stirred for 3 hours at 120° C., and the morpholinehydrobromide filtered out, and excess morpholine distilled off.

Toluene 100 ml was added, and the solution was washed with water, andthen extracted with aqueous 3N HCL solution. The HCl solution wasneutralized by adding 3N NaOH solution, and then extracted with toluene.The toluene solution was washed with water, and then toluene wasdistilled off. The residue was recrystallized from ethanol. The desiredproduct, melting at 164°-166° C., was obtained.

If desired, the photoinitiators of this invention can be combined withsecondary and tertiary alkyl and hydroxyalkyl amines, which act asaccelerators.

Such amines include, for example, triethanolamine, triisopropanolamine,methyl-diethanolamine, octyl-diethanolamine, dodecyl-diethanolamine,octadecyl-diethanolamine, dibutyl-ethanolamine, dioctyl-ethanolamine,methyl-hydroxyhexyl-octanolamine, diethanolaniline, diethanolamine,methyl-ethanolamine, butylethanolamine, dodecyl-ethanolamine,tetrahydroxyethyl-hexamethylenediamine, triethylamine, tributylamine,dimethylaminopropylamine, dimethylaniline, diethylamine, dibutylamine,dioctylamine, tetramethyl-ethylenediamine and Michler's ketone. Amongthese amines, the alkanolamines are preferable, due to their excellenteffect.

The photoinitiators of this invention can polymerize or harden anyunsaturated compounds containing at least one ethylenic double bond inthe molecule, as monomers or prepolymers.

Such unsaturated compounds include, for example, unsaturated carboxylicacids, such as acrylic acid, methacrylic acid, itaconic acid, crotonicacid and maleic acid; esters or amides of such unsaturated carboxylicacids with mono- or poly-hydric alcohols or amines; methacrylates ofpolyhydroxyesters obtained from polyol and polycarboxylic acid;unsaturated polyesters or polyamides obtained from unsaturateddicarboxylic acids and polyols or polyamines; urethane acrylatesobtained from urethane compounds containing isocyanate groups preparedfrom diisocyanates with polyols and hydroxyalkyl methacrylates;epoxyacrylates obtained from polyglycidyl ethers of bisphenols orpolyols and methacrylic acid; polyesters obtained from glycidylmethacrylate and dicarboxylic acid; dimethacryl modified polyesters orpolyamides obtained from polyesters or polyamides containing carboxylgroups and glycidyl methacrylate.

The unsaturated esters can be derived from mono- or polyhydric alcoholshaving from one to six hydroxyl groups and from one to eighteen carbonatoms, in an open aliphatic chain or carboxylic or heterocyclic ring,including methanol, ethanol, butanol, allyl alcohol, octanol, ethyleneglycol, diethylene glycol, triethylene glycol, propylene glycol,neopentyl glycol, 1,4-butanediol, 1,3-butanediol, 1,6-hexanediol,glycerine, trimethylolpropane, trimethylolethane, tris(2-hydroxyethyl)isocyanurate, pentaerythritol, diglycerine, ditrimethylolpropane anddipentaerythritol; mono- or poly-amines include ammonia, methyl amine,butylamine.

The unsaturated amides can be derived from mono or polyamino amineshaving from one to six amino groups and from one to eighteen carbonatoms in an open aliphatic chain or carboxylic or heterocyclic ring,including octylamine, diethylamine, dibutylamine, ethylenediamine,diethylenetriamine, piperazine, 1,6-hexamethylenediamine and melamine.

Preferred examples of esters and amides of unsaturated carboxylic acidsinclude methyl-, ethyl-, butyl-, isooctyl- and 2-hydroxyethyl-acrylate,methyl- and ethyl-methacrylate, ethylene glycol diacrylate, triethyleneglycol diacrylate, 1,4-butanediol diacrylate, trimethylolpropanetriacrylate, pentaerythritol triacrylate, pentaerythritol tetraacrylate,tris-(2-hydroxyethyl) isocyanurate triacrylate, dipentaerythritoltetraacrylate; dipentaerythritol, hexaacrylate, glycerine diacrylate,triethylene glycol dimethacrylate, pentaerythritol trimethacrylate,ethylene glycol dicrotonate, diallyl fumarate,bis(acryloyloxyethylphenyl)-propane, acrylamide, methacrylamide,ethylene-bis-(acrylamide), hexamethylene-bis-(acrylamide) andmethylene-bis-(acrylamide).

Examples of other photopolymerizable compounds are diallyl phthalate,diallyl malonate, divinyl phthalate, vinyl acetate, isobutyl vinylether, ethylene glycol divinyl ether, styrene, acrylonitrile, triallylisocyanurate and triallyl phosphate.

The photoinitiators of this invention are effective in small amounts,and the amount is not critical, but is selected to give the desired rateof photopolymerization according to the unsaturated compound used. Goodresults are obtained in amounts within the range from about 0.1 to about20 parts by weight, based on 100 parts of above unsaturated compound.More than this can be used.

The photopolymerizable systems comprising the photoinitiator andunsaturated compound can contain other conventional additives, such asheat polymerization inhibitors, colorants or pigments, plasticizers,surface-protecting agents, and lubricants.

Heat-polymerization inhibitors include hydroquinone, p-methoxyphenol,pyrogallol, catechol, 2,6-di-t-butyl-p-cresol, β-naphthol, andt-butylhydroquinone.

Colorants or pigments include carbon black, metal (silver, aluminum)powder, chrome yellow, titanium white, talc, alumina, Milori Blue,Chrome Vermillion, Hansa yellow, Benzidine Yellow, Vulcan Orange,Permanent Orange, Lake Red C, Brilliant Carmine B, Rhodamin Lake,Eosine, Victoria Blue Lake, Phthalocyanine Blue, Phthalocyanine Green,Quinacridone Red, Dioxazine Violet and Methyl Violet.

The polymerization is carried out in the usual way, by irradiation withlight. Suitable light sources are, for example, carbon arc, mercury highpressure lamp, xenon lamp, metal halide lamp, tungsten lamp, argon laserand helium-cadmium laser.

The following examples represent preferred embodiments ofphotopolymerization systems utilizing photoinitiators of formulae (I),(II) and (III). All parts indicated are by weight.

EXAMPLES 1 TO 9

A resin mixture composed of 75 parts of NK ester U-108A (urethaneacrylate (m.w.=1500) available from Shinnakamura Kagaku), 25 parts of1,6-hexanediol diacrylate and 0.5 part of photoinitiator as indicated inTable I was applied to paper using a film-drawing device to a thicknessof 50 mμ. The films were irradiated with a mercury high pressure lamp(Toshiba; Model H-400P) for 15 seconds (distance=12 cm). The pencilhardness of the hardened films was measured, and the results are shownin Table I.

                  TABLE I                                                         ______________________________________                                        Example No.                                                                            Photoinitiator      Pencil hardness                                  ______________________________________                                        Control 1                                                                              Benzophenone        6B                                               Control 2                                                                              1-(2-methyl-2-morpholino-                                                                         3H                                                        propionyl)-4-methylthiobenzene                                       Example 1                                                                              3-(2-methyl-2-dimethylamino-                                                                      8H                                                        propionyl) carbazole                                                 Example 2                                                                              3-(2-methyl-2-morpholino-                                                                         9H                                                        propionyl)-9-methylcarbazole                                         Example 3                                                                              3-(2-methyl-2-morpholino-                                                                         9H                                                        propionyl)-9-butylcarbazole                                          Example 4                                                                              3-(2-methyl-2-dibutylamino-                                                                       7H                                                        propionyl)-6-chlorocarbazole                                         Example 5                                                                              3-(2-methyl-2-morpholino-                                                                         6H                                                        propionyl)-9-acetylcarbazole                                         Example 6                                                                              3-(2-methyl-2-methoxypropionyl)-                                                                  7H                                                        9-methylcarbazole                                                    Example 7                                                                              3-(1-hydroxycyclohexanoyl)-9-                                                                     8H                                                        butylcarbazole                                                       Example 8                                                                              1,6-bis(9-methyl-3- 8H                                                        carbazolyl)-2,5-bis                                                           (diethylamino)hexane-1,6-dione                                       Example 9                                                                              2,7-bis(1-(9-butyl-3-                                                                             7H                                                        carbazolyloyl) isopropyl)-                                                    2,7-dimethyl-3,6-dioxaoctanol                                        ______________________________________                                    

The superior hardness when the photoinitiator of the invention was usedis apparent from the results.

EXAMPLES 10 TO 18

A resin mixture composed of 80 parts of NK-ester UVX-2 (epoxy acrylate(m.w.=1500) available from Shinnakamura Kagaku), 20 parts oftrimethylolpropane triacrylate, 70 parts of TiO₂, 6 parts ofN-methyldiethanolamine and 6 parts of photoinitiator as indicated inTable II was applied to paper using a film-drawing device to a thicknessof 10 mμ. These films were irradiated with a mercury high pressure lamp(Toshiba; Model H-400P) for 30 seconds (distance=15 cm). The pencilhardness of the hardened films was then measured, and the results areshown in Table II.

                  TABLE II                                                        ______________________________________                                                                        Pencil                                        Example No.                                                                            Photoinitiator         hardness                                      ______________________________________                                        Control 1                                                                              Benzophenone           5B                                            Control 2                                                                              1-(2-methyl-2-morpholino                                                                             HB                                                     propionyl)-4-methylthiobenzene                                       Example 10                                                                             3-(2-methyl-2-morpholino-                                                                            9H                                                     propionyl)-9-methylcarbazole                                         Example 11                                                                             3,6-bis(2-methyl-2-morpholino-                                                                       8H                                                     propionyl)-9-methylcarbazole                                         Example 12                                                                             3-(2-methyl-2-morpholino-                                                                            9H                                                     propionyl)-9-butylcarbazole                                          Example 13                                                                             3-(2-methyl-2-diethanolamino-                                                                        8H                                                     propionyl)-4-methylcarbazole                                         Example 14                                                                             3,6-bis(2-methyl-2-morpholino-                                                                       7H                                                     propionyl)-9-benzoylcarbazole                                        Example 15                                                                             3-(2-methyl-2-allyloxypropionyl)-                                                                    6H                                                     9-methylcarbazole                                                    Example 16                                                                             1,4-bis(9-butyl-3-carbazolyl)-2,3-                                                                   7H                                                     dihydroxybutane-1,4-dione                                            Example 17                                                                             1,4-bis(1-(9-methyl-3-carbazolyloyl)                                                                 8H                                                     isopropyl) piperidine                                                Example 18                                                                             N,N'-dimethyl-N,N'-bis(1-(9-                                                                         7H                                                     methyl-3-carbazolyloyl) isoporopyl)                                           ethylenediamine                                                      ______________________________________                                    

The superior hardness when the photoinitiator of the invention was usedis apparent from the results.

EXAMPLES 19 TO 26

A resin mixture composed of 80 parts of NK-ester U-108A (urethaneacrylate (m.w.=1500) available from Shinnakamura Kagaku), 20 parts oftrimethylolpropane triacrylate, 22 parts of phthalocyanine blue(Dainichiseika; SR-5020), 7 parts of ethyl-4-dimethylaminobenzoate and 7parts of photoinitiator as indicated in Table III was applied to paperusing a film-drawing device to a thickness of 10 mμ. These films wereirradiated with a mercury high pressure lamp (Toshiba; Model H-400P) for30 seconds (distance=15 cm). The pencil hardness of the hardened filmswas measured, and the results are shown in Table III.

                  TABLE III                                                       ______________________________________                                                                        Pencil                                        Example No.                                                                            Photoinitiator         hardness                                      ______________________________________                                        Control 1                                                                              Benzophenone           2B                                            Control 2                                                                              1-(2-methyl-2-morpholino-                                                                            2H                                                     propionyl)-4-methylthiobenzene                                       Example 19                                                                             3-(2-methyl-2-dimethylamino-                                                                         6H                                                     propionyl) carbazole                                                 Example 20                                                                             3-(2-methyl-2-morpholino-                                                                            7H                                                     propionyl)-9-methylcarbazole                                         Example 21                                                                             3-(2-methyl-2-morpholino-                                                                            7H                                                     propionyl)-9-butylcarbazole                                          Example 22                                                                             3-(2-ethyl-2-piperidino-                                                                             6H                                                     propionyl)-9-butylcarbazole                                          Example 23                                                                             3-(2-methyl-2-piperadino-                                                                            5H                                                     propionyl-6-nitro-9-methylcarbazole                                  Example 24                                                                             3-(2-methyl-2-hydroxy- 6H                                                     propionyl)-9-methylcarbazole                                         Example 25                                                                             1,4-bis(9-butyl-3-carbazolyl)-                                                                       5H                                                     2,3-dihydroxybutane-1,4-dione                                        Example 26                                                                             1,4-bis(1-(9-methyl-3- 5H                                                     carbazolyloyl) isopropyl)                                                     piperidine                                                           ______________________________________                                    

The superior hardness when the photoinitiator of the invention was usedis apparent from the results.

Having regard to the foregoing disclosure, the following is claimed asinventive and patentable embodiments thereof:
 1. A photopolymerizableprepolymer composition comprising a photoinitiator for thephotopolymerization of unsaturated compounds having one of the formulae:##STR9## wherein: R₁ and R₂ are each alkyl having from one to abouteighteen carbon atoms; or are taken together to form alkylene havingfrom about two to about twelve carbon atoms;R₃ is selected from thegroup consisting of hydrogen; alkyl having from one to about eighteencarbon atoms; and acyl having from about two to about twelve carbonatoms; X₁ is --OR₅ or ##STR10## in which R₅ is selected from the groupconsisting of hydrogen; alkyl and alkenyl having from one to abouteighteen carbon atoms; and R₆ and R₇ are selected from the groupconsisting of alkyl; hydroxyalkyl having from one to about eighteencarbon atoms; and R₆ and R₇ taken together to form alkylene having fromabout two to about twelve carbon atoms; and oxadialkylene andiminodialkylene having from four to about twenty-four carbon atoms; Y₁is hydrogen or ##STR11## Y₂ is selected from the group consisting ofhydrogen, halogen, and nitro; R₄ is a direct linkage (--) or alkylenehaving from about two to about twelve carbon atoms; X₂ is --O--R₈ --O--or ##STR12## in which: R₈ is alkylene having from about two to abouttwelve carbon atoms;R₉ and R₁₀ are alkyl having from one to abouteighteen carbon atoms; or are taken together to form alkylene havingfrom about two to about twelve carbon atoms;and an unsaturated compoundhaving at least one ethylenic double bond that is photopolymerizable toform a polymer.
 2. A photopolymerizable prepolymer compositioncomprising a photoinitiator according to claim 1 in which theunsaturated compound is a monomer.
 3. A photopolymerizable prepolymercomposition comprising a photoinitiator according to claim 1 in whichthe unsaturated compound is a prepolymer.
 4. A photopolymerizableprepolymer composition comprising a photoinitiator according to claim 1in which the unsaturated compound is an unsaturated carboxylic acidester.
 5. A photopolymerizable prepolymer composition comprising aphotoinitiator according to claim 4 in which the unsaturated carboxylicacid is acrylic acid ester.
 6. A photopolymerizable prepolymercomposition comprising a photoinitiator according to claim 4 in whichthe unsaturated carboxylic acid is methacrylic acid ester.
 7. Aphotopolymerizable prepolymer composition comprising a photoinitiatoraccording to claim 1 also containing a secondary or tertiary alkyl orhydroxyalkyl amine.
 8. A process for photopolymerizing an unsaturatedcompound having at least one photopolymerizable ethylenic double bond,which comprises initiating the unsaturated compound with actinic lightin the presence of a photoinitiator for the photopolymerization ofunsaturated compounds having one of the formulae: ##STR13## wherein: R₁and R₂ are each alkyl having from one to about eighteen carbon atoms; orare taken together to form alkylene having from about two to abouttwelve carbon atoms;R₈ is selected from the group consisting ofhydrogen; alkyl having from one to about eighteen carbon atoms; and acylhaving from about two to about twelve carbon atoms; X₁ is --OR₅ or##STR14## in which R₅ is selected from the group consisting of hydrogen;alkyl and alkenyl having from one to about eighteen carbon atoms; and R₆and R₇ are selected from the group consisting of alkyl; hydroxyalkylhaving from one to about eighteen carbon atoms; and R₆ and R₇ takentogether to form alkylene having from about two to about twelve carbonatoms; and oxadialkylene and iminodialkylene having from four to abouttwenty-four carbon atoms; Y₁ is hydrogen or ##STR15## Y₂ is selectedfrom the group consisting of hydrogen, halogen, and nitro; R₄ is adirect linkage (--) or alkylene having from about two to about twelvecarbon atoms; X₂ is --O--R₈ --O-- or ##STR16## in which: R₈ is alkylenehaving from about two to about twelve carbon atoms;R₉ and R₁₀ are alkylhaving from one to about eighteen carbon atoms; or are taken together toform alkylene having from about two to about twelve carbon atoms.
 9. Aphotopolymerizable prepolymer composition according to claim 1, in whichthe photoinitiator has formula I.
 10. A photopolymerizable prepolymercomposition according to claim 1, in which the photoinitiator hasformula II.
 11. A photopolymerizable prepolymer composition according toclaim 1, in which the photoinitiator has formula III.